1. Field of the Invention
The present invention relates to a method of manufacturing a silicon monocrystal in accordance with the Czochralski (CZ) method without performing a so-called necking operation. Further, the present invention relates to a seed crystal for use in the manufacturing method, as well as to a method of manufacturing such a seed crystal.
2. Description of the Related Art
In the manufacture of a silicon monocrystal using the Czochralski (CZ) method, monocrystalline silicon has conventionally been used as a seed crystal. A silicon monocrystalline ingot is grown by bringing the seed crystal into contact with the silicon melt and pulling the seed crystal slowly while it is rotated. At this time, operation for forming a neck portion (necking operation) is performed in order to eliminate dislocations generated in the seed crystal at a high density as a result of thermal shock arising when the seed crystal is brought into contact with the silicon melt. Subsequently, the diameter of the crystal is increased to a desired diameter, and the silicon monocrystalline ingot is then pulled. The necking operation has been well known as a "Dash Necking Method," and use of this method has been a common practice in the case where a silicon monocrystal ingot is pulled in accordance with the CZ method.
Specifically, as shown in FIGS. 4A and 4B, a conventional seed crystal is formed into a cylindrical shape having a diameter of about 8-20 mm or into a prismatic shape having sides of about 8-20 mm, and a cutaway portion is formed for attachment to a seed crystal holder. The tip or lower end of the seed crystal, which end first comes into contact with silicon melt, is formed to have a flat surface. In order to safely pull a heavy monocrystalline ingot while sustaining the weight of the ingot, the seed crystal must have a dimension in the above-described range.
However, since the seed crystal having the above-described shape and dimension has a large heat capacity at the tip end which comes into contact with silicon melt, a steep temperature gradient is generated instantaneously within the crystal when the seed crystal comes into contact with the melt, so that slip dislocation is generated at a high density. Therefore, the above-described necking operation is required for growing a monocrystal while eliminating the dislocation.
In the Dash Necking Method, after a seed crystal is brought into contact with silicon melt, the diameter of the crystal is reduced to about 3 mm before being increased, in order to form a neck portion to thereby eliminate dislocation induced from slip dislocation generated in the seed crystal and thereby to be grown a dislocation-free monocrystal.
However, in such a method, even when conditions for the necking operation are selected appropriately, the diameter of the crystal must be decreased to 5-6 mm or less in order to eliminate the dislocation. In such a case, the strength becomes insufficient to support a monocrystalline ingot whose weight has been increased with a recent increase in the diameter thereof, resulting in a high risk of fracture of the neck portion during the course of pulling of the monocrystalline ingot. This may result in serious accidents such as a drop of the monocrystalline ingot.
To solve the above-described problem, there has been developed a method of pulling a recent heavy large-diameter monocrystalline ingot through use of a crystal holding mechanism (see, e.g., Japanese Patent Publication (kokoku) No. 5-65477).
In this method, a growing monocrystalline ingot is held directly and mechanically, in consideration of the above-described knowledge that necking operation is indispensable for elimination of dislocation and that therefore the strength of the neck portion cannot be increased.
However, in such a method, a monocrystalline ingot--which is grown at a high temperature while being rotated--is directly held, an apparatus used for implementing the method becomes complicated and expensive and also raises a problem related to heat resistance. Further, in practice, it is extremely difficult to hold a growing crystal without generating vibration therein, so that the growing crystal may become a polycrystal. Moreover, since a complicated apparatus including mechanisms for rotation, sliding, and other motions must be disposed above a silicon melt of high temperature, there arises a problem that the crystal may be contaminated by heavy metal impurities.
In order to solve these problems, that applicant of the present invention has proposed various inventions such as those disclosed in Japanese Patent Application Laid-Open (kokai) No. 5-139880 and Japanese Patent Application No. 8-87187. According to these inventions, the tip end of a seed crystal is formed into a wedge shape or is formed to have a hollow portion in order to reduce, to the extent possible, slip dislocation which would otherwise be generated when the seed crystal comes into contact with silicon melt. These inventions enable elimination of dislocation, even when the neck portion is formed to have a relatively large diameter, thereby increasing the strength of the neck portion.
Although the methods according to the inventions can increase the strength of the neck portion to some degree through an increase in the diameter of the neck portion, the methods still require necking operation, resulting in formation of a neck portion having slip dislocation. Therefore, in some cases, the strength of the neck portion of a monocrystalline ingot manufactured in accordance with either of these methods becomes insufficient for pulling the ingot if the monocrystalline ingot has a weight of 150 Kg or more as a result of recent increases in the diameter and length thereof. Accordingly, the methods do not thoroughly solve the problems involved in the prior art methods.
In order to solve the above-described problems, the applicant of the present application has successfully developed a method of manufacturing a silicon monocrystal, which method can make a growing crystal monocrystalline without performance of a necking operation for forming a neck portion, which would cause a problem in terms of strength, thereby enabling a heavy silicon monocrystal having a large diameter and length to be pulled quite simply while eliminating the necessity of using a complicated apparatus such as a crystal holding mechanism. The applicant of the present application has also developed a seed crystal used in the method (Japanese Patent Application No. 9-17687).
In this method, the tip end of a crystal used as a seed crystal, which end comes into contact with silicon melt, has a sharp-pointed shape or a truncation thereof. After the tip end of the seed crystal is gently brought into contact with the silicon melt, the seed crystal is lowered at a low speed in order to melt the tip end portion of the seed crystal until the thickness of the tip portion increases to a desired value. Subsequently, the seed crystal is pulled slowly in order to grow a silicon monocrystalline ingot having a desired diameter without a necking operation being performed.
This method can thoroughly solve the problems involved in formation of a neck portion because the method does not includes necking. Therefore, the above-described method is considerably excellent. However, a subsequent running test revealed that, depending on the shape of a seed crystal and the method of manufacturing the same, dislocation is likely to be generated in the seed crystal when the tip end of the seed crystal comes into contact with silicon melt and melts, and that generation of such dislocation makes subsequent growth of the monocrystal difficult, resulting in a decreased success ratio in obtaining a dislocation-free monocrystal.
Further, in this method, if dislocation is once generated in the seed crystal, the pulling operation cannot be performed again unless the seed crystal is replaced with a new one. Therefore, increasing the success ratio is especially important.
Further, even when no dislocation is generated when the tip end of the seed crystal comes into contact with silicon melt, generation of slip dislocation sometimes occurs when the thickness becomes equal to or greater than a certain value (about 5 mm in diameter) during the operation of melting the tapered portion of the tip end of the seed crystal to obtain a desired thickness. Therefore, in some cases, the ratio of success in making crystals dislocation free (hereinafter referred to as the "dislocation-elimination success ratio") is not high, and a sufficient degree of reproducibility cannot be obtained.